The so-called SLM process provides for the selective laser melting of metal powder to build layer by layer the intended item.
Each section of the item is built by means of consecutive scan of the laser beam, which determines the linear melting of the metal powder, with the consequent progressive formation of a compact layer of alloy.
One of the main problems of this manufacturing process is the reflection of the electromagnetic radiation, which causes a poor absorption of energy and a reduced capacity of melting of the metallic particles.
This drawback is particularly felt in the jewellery art, due to the high reflectivity of precious alloys, particularly the gold alloys, a material that has an electric resistivity considerably higher than silver.
To avoid this drawback, additional surface treatments (oxidation, paint, etc.) are performed, leading to the formation of thin layers having an absorption higher than that of the substrate, which is then heated in an indirect way.
Furthermore, the high reflectivity of precious alloys involves the formation of high surface roughness, constituted of the projection of particles of metal powder above the layer of construction, the particles being also responsible of the formation of undesired surface bulges and porosities.
From the international application WO2005/025783 a powder mixture is known susceptible for manufacturing jewellery items by SLS. This mixture is composed of a basic material and a flux material that are sintered together.
It is clear that since the starting material is strongly inhomogeneous, the jewellery item thus manufactured has very strong irregularities in its chemical-physical structure.
Furthermore, this document suggests the use of a single powder consisting of particles having a completely fusible surface layer and an only partially fusible core.